We first briefly recall some properties of primordial neutrinos from standard
cosmology. At temperatures, \(T\) , small compared with the muon mass but well above 1 MeV, muons and tau are frozen out
and the only charged leptons present are electrons and positrons;
neutrinos are held in thermal equilibrium with the ambient plasma
through neutral and charged current interactions.
Owing to the charged current interaction for
\(\nu _e\)  on electrons and \(\bar{\nu }_e\)  on positrons, the reaction
cross sections for electron neutrinos are larger
than for muon and tau neutrinos. An important consequence is that muon
and tau neutrinos decouple from the plasma at an earlier time and at
a higher temperature than electron neutrinos.
As estimated in Ref. [1]}, \(\nu _\tau \)  and \(\nu _\mu \)  freeze
out at temperature \(T_\mu \sim 1.5\)  MeV, while \(\nu _e\)  freeze out
at temperature \(T_e\sim 1.3\)  MeV. However, the temperature differences
at freezeout do not effect the present temperature, \(T_{\nu 0}= 1.945 \pm 0.001\) K = \((1.676\pm 0.001)\times 10^{-4}\)  eV, of the various neutrino species (a
factor \((11/4)^{1/3}\)  smaller than that of the cosmic
microwave background).
